The invention relates to methods of making graft copolymers for use in coating compositions, and more particularly to improved methods of making acrylic graft copolymers having graft moieties such as polyurethane, carbamate, and groups convertible to carbamate.
Waterborne acrylic graft copolymers provide a variety of advantages in a number of applications. However, prior art methods of making acrylic graft copolymers for use in water based systems encounter a number of problems.
For example, many prior art acrylic graft copolymers were prepared by first preparing an unsaturated monomer having a graft moiety appended thereto. This monomer was then emulsified, often by high shear or high stress means. The emulsion was then subjected to free radical polymerization.
In many instances, however, the preparation of the unsaturated monomer having the graft moiety appended thereto was particularly challenging. The temperature required for reaction between a functional group of the unsaturated monomer and a functional group of the graft moiety often resulted in gellation of the unsaturated monomer. Another frequently encountered drawback was poor conversion and low yields. As a result, such processes have been disfavored in commercial manufacturing operations.
Thus, it would be advantageous to provide a method of manufacturing a variety of acrylic graft copolymers which does not encounter the gellation and low conversion problems of the prior art. It would be an advantage if such a method could be used with a variety of graft moieties useful in acrylic graft copolymers.
Accordingly, it is an object of the invention to provide a method of making waterborne acrylic graft copolymers which is essentially a one step method and which does not have the disadvantages of the prior art processes.
More particularly, it is an object of the invention to provide a method of making waterborne acrylic graft copolymer which contain polyurethane or carbamate group containing graft moieties.
These and other objects have been met with the method of the invention. In particular, the invention provides a method of making graft polymers comprising providing an emulsion comprising at least one or more first monomer (a), at least one or more second monomer (b), and at least one graft material (c). The emulsion is subjected to conditions such that it undergoes simultaneous free radical polymerization and grafting. One or more first monomer (a) is unsaturated and is free of any functional groups reactable with functional groups Fb or Fc. Monomer (a) may optionally comprise a functional group Fa which is not reactive with either functional group Fb or Fc. One or more second monomer (b) is unsaturated and comprises a functional group Fb reactive with functional group Fc. Graft material (c) comprises a compound free of unsaturation and comprising a functional group Fc reactive with functional group Fb, and a graft moiety. The saturated graft material (c) may be monomeric, oligomeric, or polymeric. Most preferred graft moieties are carbamate groups, groups convertible to carbamate and polyurethane polymers.